Land Use Planning to Avert the Migration Oriented Watershed Degradation
Sah, Bhuwneshwar Prasad and Shimizu, Eihan
Regional Planning & Surveying Laboroatroy/Department of Civil engineering
The University of Tokyo, Bunkyo-ku Hongo, Tokyo 113, Japan
Tel.: +81-3-3812221111 Ext. 6129, Fax 81-3-56897290
E-mail:sah@planner.t.u-tokyo.ac.jp,
shimizu@planner.t.u.-tokyo.ac.jp
Abstract
Like in other developing countries, Nepal is also suffering from severe watershed degradation. In Nepalese contest, its main reason is population pressure due to higher population growth rate and in-migration. The fertile valley of Trijuga River in the Udapur district of Eastern Nepal is on example of such phenomenon where about 48 percent of household has migrated-in during last 15 years and most of them have encroached the forestland enhancing soil erosion. Under this context, the present study has been carried out to approach for better land use plan by developing five possible scenarios. The result has shown that 50% reduction in soil erosion. Under this context, the present study has been carried out to approach for better land use plan by developing five possible scenarios. The result has shown that 50% reduction in soil erosion. rate can be achieved by readjusting the land use and reallocation of some household. The land use definition, with clear and strong institutional support, is desirable here.
1.Introduction
Degradation of watershed is a common phenomenon around the world. There are several reasons for such degradation, but most important is improper utilization of watershed resources without any the degradation is generally defined in terms of forest loss and increase in soil erosion rate (DSCWM, 1983). The population pressure, which is resulted from higher rate of population growth (2.25 percent per year, CBS 1994A) and internal migration, for arable land is tremendously high in Terai and valley of mountain area. Forest encroachment, especially in slope, acts double role, that is forest loss followed enhanced soil erosion.
During last few years, flow of migration of people from hilly area to Terai and fertile valley has remained very high. This is, basically, due to the improvement of accessibility and flow of information. Lack of institutional capabilities and suitable land use plan with ample definition of land use encourages the migratory people to encroach in forest on their will. The Trijuga watershed of Eastern Nepal is one good example of such phenomenon. Estimating migration rate through survey, Sah 91996) has reported that 48 percentage of household has been migrated-in during the period of 15 years (between 1975 to 1990) and most of them have encroached in the forestland . This migration and faulty land use has resulted in decrease in forest by 7.35 percent and increase in soil erosion by 44 percent.
Although, there might be other approaches to mitigate the watershed degradation, land use planning approach has been adapted in this study. The objection were:
-
Prapare different scenario by using land use/cover, soil erosion rate, shope gradient and
population of the watershed.
-
Propose land use plan with desirable/sustainable soil erosion rate.
2 The Study Area:
The study area, Trijuga watershed, lies in Udapur district in the eastern Nepal. It is located between 26o 42' and 26o 59'N latitude and 86o 33' 46" and 86o 59' 48" E longitude. Trijuga, the main river of watershed, drains in to the Koshi River and comprises of several rivulets and rivers due to ruggee topography in the area. Total area of the watershed is 732 sq. km and altitude varies from 75 to 1942 meters. The climate varies from tropical in low-tying valley to sub-tropical in the higher elevation. The average annual rainfall and temperature is 1942 mm and 20oC (WEC, 1982), respectively. More than 70 percent rainfall is concentrated from May to October. Forest cover is nearly 58 percent which is dominated by tropical Sal (Shorea robustra) followed by agriculture in 24 percent area in past, the watershed was dominated by indigenous ethnic people, Tharu, but now dominated by migratory people from other parts of Nepal, especially from eastern hill and mountain Area. Agriculture is main source of income and livelihood of the people.
3. Methodology:
Land use change, soil erosion rate and migration data that estimated by Sah (1996) were used as input for this study. The land use change was derived by comparing the old land use map of 1978 extracted from air photograph, with the land use map of 1991 extracted from Landsat TM. The soil erosion rate was estimated by employing University soil, Los Equation (USLE) model. (Schwab et al. 1993). Similary, houselold survey was conducted for the estimation of migration.
The frend of land use and soil erosion rate change for the period of 13 years (from 1978 to 1991) were interpreted and evaluated by considering the permissible limit for the sustainable use of watershed resources. For this, two crucial factors, viz., upper permissible limit of soil erosion rate 910 to 20 t/ha/yr.) and upper limit of slope for cultivation (20 to 30 percent) were taken into consideration. These are exclusively considered as most important while talking about sustainability of land use (Huang & Wu, 1995, DSCWM/HMG, 1992, Schertz, 1993, Williams et al., 1993, Sharma, 1981). The existing in -migration rate, distribution of population and agricultural land, and suitable land for agriculture of the watershed were reexamined. Based on the evaluation of these factors, the desirable land use was simulated with following five scenarios:
-
Converting Shrubs into Forestland.
- Converting Degraded Forest into Forestland.
- Converting shrubs and Degraded Forest beyond 20 percent slope in a Forestland.
- Restricting the Agriculture below 20 percent of slope. The Agriculture Land above 20 percent of slope will be converted in to Forest and equivalent of those lands will be substituted from Degraded Forest land of below 5 percent slope.
- Combination of (iii) and (iv)
Furthermore, while developing the above scenarios, the land sensitivity (Sah et al., 1997), that is relation between soil erosion rate, land use/cover type and slope gradient, was also given due consideration.
Finally, the better and use plan for sustainable use of watershed was taken and recommended for the planner policy markers. The overall methodology has been shown in Figure 1.
4. Data Used:
- Satellite data: Landsat TM path/row 140041, acquired on 21 Dec. 1991.
- Aerial photograph: Scale 1:50,000, Nov., 1978.
- Reconnaissance maps: Topographic map: 1:25,000, 1995, Land use map: 1:50,000, 1982, Land System map: 1:50,000, 1994, and Political map: 1:250,000
5. Result and Discussion:
5.1 The Watershed Degradation and Present Situation:
As shown in Table 1, during the period of 13 years (between 1978 to 1991), the forest area has decreased from 65.21% to 57.86%, that is, rate of forest destruction remained 0.57% per year. Similarly, during the same period, rate of soil erosion has increased from 12 t/ha/yr. To 18 t/ha/yr. In 1991 (Table 1). Thus neither the forest area could be maintained nor the rate of soil erosion has remained constant. The distribution of land use and soil erosion rate of year 1991 is shown in Map 1 and 2 respectively.
Table 1: Land USE/Cover and soil Erosion Rate of Trijuga Watershed
| Land cover |
year |
Land Use Change 1978-91(%) |
Ave. soil loss (t/ha/yr) |
%Contribution to soil loss |
| 1978 (%) |
19991 (%) |
1978 |
1991 |
1978 |
1991 |
| Shrube |
2.27 |
3.45 |
1.18 |
123.36 |
180.24 |
22.47 |
34.61 |
| Degraded forest |
4.19 |
8.90 |
4.71 |
10.96 |
10.53 |
3.68 |
5.21 |
| Forest |
65.21 |
57.86 |
-7.35 |
1.72 |
1.85 |
8.99 |
5.96 |
| Agriculture |
22.49 |
23.66 |
1.17 |
35.99 |
41.20 |
64.86 |
54.21 |
| River |
5.84 |
6.14 |
0.30 |
0.00 |
0.00 |
0.00 |
0.00 |
| Total |
100 |
100 |
0.00 |
12.48 |
17.98 |
100.00 |
100.00 |
Degraded forest < 40% & Forest=> 40% canopy cover
Forestloss = 7.35%/13 years =0.57 % per year
Soil loss increment (17978.91)=(17.98-12.48)/12.48*100 =44%
